The present invention relates to droplet generation and, more particularly, to the generation of liquid droplets from a membrane having one or more apertures.
In our European Patent EP-B-0615470, we describe an apparatus of this general type and there are numerous other examples of droplet generators of this type, see in particular, our European Patent EP-B-0732975.
Other prior art of possible relevance includes: EP-A-0 432 992, GB-A-2 263 076, EP-A-0 516 565, U.S. Pat. No. 3,738,574, EP-A-0 480 615, U.S. Pat. No. 4,533,082 and U.S. Pat. No. 4,605,167.
In our EP-B-0732975 we describe and claim a liquid droplet spray device comprising a perforate membrane; an actuator, for vibrating the membrane; and means for supplying liquid to a surface of the membrane, characterised in that perforations in the membrane have a larger cross-sectional area at that face of the membrane away from which liquid droplets emerge than at the opposite face of the membrane.
According to EP-B-0732975, such a device is a perforate membrane capillary wave droplet generator believed to be excited and to pin capillary waves within the small orifice of the membrane perforations. It employs a perforate membrane designed to spatially match the excited capillary wave field (i.e. one or more capillary waves pinned within each orifice), so that the coupling efficiency of energy transferred to the oscillating capillary waves is greatly enhanced over free-surface capillary wave generators. Thus, significantly lower power and lower cost devices can be employed to generate droplets using the capillary wave approach. However, even with the enhanced energy coupling provided in such devices, devices of this type together with the drive electronics needed to excite their operation still consume up to 10 W of power to operate in the 250 kHz-8 MHz frequency region to enable the production of small water droplets between 1 and 10 xcexcm in diameter. It is desirable to create such small droplets, for example, in order to practice electrophotography as described in our International patent application no. PCT/GB96/01671.
The present invention is aimed particularly, but not solely, at reducing the power consumption necessary to produce small droplets using a device of the vibratory perforate membrane type. Drive electronics operating at higher frequencies in the range 250 kHz to 8 MHz are also generally more expensive than drive electronics otherwise similar in nature but operating at lower frequencies. The present invention, in allowing lower frequency operation of vibrating perforate membrane droplet generators, therefore also has utility in reducing the overall cost of such droplet generators, particularly in the production of small droplets relative to those of capillary wave type described above.
According to the present invention, there is provided a method of generating liquid droplets from an aperture in a membrane, the aperture having a diameter xcfx86 at one face at most as large as its diameter at the other face, the method comprising
providing a liquid to the one face of the membrane adjacent the aperture, the liquid having a surface tension "sgr" and density xcfx81;
applying a pressure bias such that the pressure of the liquid at the one face of the membrane is lower than the pressure immediately adjacent to the opposing face of the membrane; and
while the bias pressure is applied, vibrating the membrane at a frequency f which is determined by the relationship:   f  ≤            (                        5          ⁢                      xe2x80x83                    ⁢          π          ⁢                      xe2x80x83                    ⁢          σ                          ρ          ⁢                      xe2x80x83                    ⁢                      φ            3                              )              1      /      2      
in order to generate droplets emergent from the other face.
The invention also includes a droplet generator for generating droplets of a liquid having a surface tension "sgr" and density xcfx81, the droplet generator comprising
a membrane having a first face and a second face, an aperture extending through the membrane from the first face to the second face, the aperture being of a diameter xcfx86 at the first face at most as large as its diameter at the second face;
means for applying a pressure bias such that the pressure of the liquid at the first face of the membrane is lower than the pressure immediately adjacent to the second face of the membrane; and
a vibration generating means for causing the membrane to vibrate at a frequency f which is determined by the relationship:   f  ≤            (                        5          ⁢                      xe2x80x83                    ⁢          π          ⁢                      xe2x80x83                    ⁢          σ                          ρ          ⁢                      xe2x80x83                    ⁢                      φ            3                              )              1      /      2      
in order to generate droplets emergent from the other face.
The invention further includes a liquid atomisation head for a droplet generator for generating droplets of a liquid having a surface tension "sgr" and density xcfx81, the liquid atomisation head comprising
a membrane having a first face and a second face, an aperture extending through the membrane from the first face to the second face, the aperture being of a diameter xcfx86 at the first face at most as large as its diameter at the second face; and
a vibration generating means for causing the membrane to vibrate at a frequency f which is determined by the relationship:   f  ≤            (                        5          ⁢                      xe2x80x83                    ⁢          π          ⁢                      xe2x80x83                    ⁢          σ                          ρ          ⁢                      xe2x80x83                    ⁢                      φ            3                              )              1      /      2      
in order to generate droplets emergent from the other face.
By xe2x80x9cliquid atomisation headxe2x80x9d is meant a device which, in use, converts liquid in contact with it into liquid droplets emergent from it. This includes aerosol generation and ink jet devices. xe2x80x9cLiquidxe2x80x9d is to be understood to include pure liquids, mixtures of liquids, solutions, and suspensions of solid particles within a liquid carrier.
According to a second aspect of the invention, the relationship in the statements of invention defined above is replaced by the relationship:   f  ≤            (                        3.6          xc3x97                                    10                              -                4                                      ·            π                                    φ          3                    )              1      /      2      
The relationship immediately above applies in the case, particularly, but not exclusively, of water, and falls wholly within the definition of the frequency relationship given previously above.
Preferably, the vibration generating means is integrally or intimately formed with the membrane.
Prior art devices of the vibrating perforate membrane type are most commonly known with perforations that decrease in size from the side of the membrane to which bulk liquid is supplied to the opposite, droplet-emergent, side. These are known as forward-tapered perforations and in devices having such forward-tapered perforations, droplet ejection consistent with the condition fxe2x89xa6(5Π"sgr"/xcfx81xcfx863)xc2xd is understood from, for example U.S. Pat. No. 5,164,740. However, droplet ejection under that condition with devices in which the perforations are tapered as in devices according to the present invention, (xe2x80x98reverse-taperedxe2x80x99), has not been previously observed. Furthermore, droplet generation in these circumstances cannot be understood from the capillary wave mechanism known from previous reverse taper devices as described, for example, in EP-B-0732975.
Ultrasonic droplet generators, which are preferably used in the present invention, typically have a number of harmonic vibration frequencies at which they will resonate. At some of these harmonic frequencies, a capillary wave may be excited within each membrane perforation or aperture. As the harmonic vibration frequency increases, so the capillary wavelength of excited capillary waves decreases and the emergent droplet diameter reduces accordingly. These features are known from the following equations:
xcfx86d≈xcex/3
and                               λ          3                =                              8            ⁢                          xe2x80x83                        ⁢            π            ⁢                          xe2x80x83                        ⁢            σ                                ρ            ⁢                          xe2x80x83                        ⁢                          f              2                                                          (        1        )            
where xcfx86d is the droplet diameter and xcex is the wavelength of the capillary wave.
According to the model presented in EP-B-0732975, an apparatus is provided with membrane orifices such that one or more such capillary waves fits within and is xe2x80x9cpinnedxe2x80x9d by the relatively large opening of a perforation at the rear face of a membrane.
In this way, an otherwise large opening at the rear face of a membrane can produce a small droplet emerging from the front face of the membrane. For a given liquid (and hence "sgr" and xcfx81) the diameter of the emerging droplet is primarily determined by the meniscus frequency at which the capillary wave is vibrating and the large opening at the rear face of the membrane serves to pin the capillary wave within it rather than determine the droplet diameter.
Conversely, as the harmonic vibration frequency decreases, the capillary wave length increases and the emergent droplet diameter increases. However, droplet generation according to the capillary wave mechanism as described in EP-B-0732975 only occurs at frequencies above that frequency at which a capillary wave length is supported by the opening at the rear face of the membrane. The present inventors have shown that droplet generation can be obtained at lower frequencies and that, in this new low-frequency regime, the droplet production mechanism deviates from the capillary wave model referred to above in equation 1. In droplet generation according to the new mechanism, in this low frequency regime, the droplet size is generally found to be of a diameter approximately equal to that of the opening in the rear face of the membrane, rather than following the frequency-dependency expressed by equation (1).
In droplet generation according to this new mechanism it is found necessary to apply a bias pressure to the liquid contacting the rear face of the membrane. The bias pressure is selected to maintain that liquid at a pressure lower than the pressure (which is typically but not necessarily atmospheric pressure) immediately adjacent the opposing face of the membrane. This bias pressure is selected to be sufficient to overcome the liquid pumping effect described in EP-B-0732975 and is preferably insufficient to draw air through the perforations into the liquid. Thus, a new droplet generating mechanism is employed, the result of which is the ability to create a given droplet diameter at a frequency much below that required by the capillary wave model.
The present invention thereby enables low frequency production of small droplets, i.e. droplets smaller in diameter that those predicted by the capillary wave model, at a given harmonic vibration frequency. This advantage is particularly marked in the production of droplets in the size range of 1 xcexcm to 10 xcexcm from such xe2x80x98reverse taperxe2x80x99 devices. This reduction in frequency reduces the electrical power needed for droplet production and the cost of drive electrodes to produce the needed motional excitation of the membrane.
It should be noted that the negative bias pressure provided in operation does not require that the liquid is brought to the membrane at a negative bias pressure, only that the liquid is subjected to a negative bias pressure at those times that droplet generation is required. For example, the liquid may be brought to the perforate membrane at a pressure equal to that immediately in front of the perforate membrane, or even brought to the perforate membrane at a higher pressure that is insufficient, of itself, to cause the liquid to flow through the perforations. Thereafter, when atomisation into droplets is required, means may be employed either to reduce the pressure (of the liquid body that then contacts those perforations) below the pressure immediately in front of the perforate membrane, or to increase the pressure of the atmosphere immediately in front of the membrane, either of which creates the same desired pressure differential. Either simultaneously with establishing that pressure differential or at a subsequent time whilst that pressure bias remains, the vibration of the membrane (in the defined frequency range) can then be excited to produce droplets.
This new droplet mechanism is much more robust in the production of atomised particle suspensions, as well as solutions, without clogging of the apertures or perforations, than prior art xe2x80x98forward taperxe2x80x99 droplet production devices such as those cited above. This applies particularly to its use with small perforations to produce small droplets such as described above. This advantage derives from the reverse taper nature of the perforations or apertures, which together with the bias pressure, described, is believed to pin the fluid meniscus at that face of the membrane to which liquid is supplied leaving the perforation itself unfilled by the liquid or suspension.